The failure of the supposed Unha-3 rocket, which the US says was actually a long-range ballistic Taepodong-2 missile, is a major setback for the North's regime.

The launch has attracted international condemnation and the UN Security Council will hold an emergency meeting later Friday.

In a broadcast on state TV, North Korea said its scientists were trying to determine why the rocket failed.

It exploded just minutes after launch, with pieces of it falling into the Yellow Sea west of the South Korean capital Seoul.

South Korea's announcement of the salvage operation came despite a warning from the North last week for it not to attempt such an operation.

"If anyone... attempts to shoot down or to salvage our space rocket debris, we would retaliate ruthlessly," Pyongyang's Committee for the Peaceful Reunification of the Fatherland, in charge of inter-Korean ties, had said.

Yonhap news agency said more than a dozen South Korean ships, many of them equipped with sonar and backed by divers, were combing the area.

"We believe some large chunks are lying on the sea bed," the spokesman said.

The depth of the sea is between 70 and 100 metres, making it feasible to retrieve large chunks of debris, Yonhap reported.

Armed response

Meanwhile, a senior South Korean defence ministry official said the North would likely stage military provocation against the South in the wake of the failed launch.

"The possibility of an additional long-range rocket launch or a nuclear test, as well as a military provocation to strengthen internal solidarity, is very high," the official told a parliamentary hearing.

But he added there were no specific indications of military actions from the North.

The failed launch is a humiliating blow for North Korea's new leader, Kim Jong-un, who launched the rocket to celebrate the 100th anniversary of the birth of his grandfather and the country's founder, Kim Il-sung.

Australia's Foreign Minister Bob Carr says it is hard to say how North Korea will behave now.

"It's very hard to judge what motivates this regime; it's a one-party Marxist-Leninist dictatorship, but it could well be the case that this failure will put pressure on the leadership to do something even wilder to maintain public support with the 25 million people of North Korea."